1. Field of the Invention
The present invention relates generally to a rare earth-doped optical amplifier, and particularly to a polymer-based erbium-doped waveguide device for use in an erbium-doped waveguide amplifier (EDWA).
2. Description of Prior Art
Optical amplifiers increase the optical output power of an end-terminal system. They can also be used as repeaters, thus allowing increased distance between end-terminal equipment. Rare earth-doped amplifiers can be stimulated to produce a laser that has the same wavelength as that of incident light. Erbium-doped amplifiers are the most established and accepted rare earth-doped amplifiers.
A type of erbium-doped amplifier called an erbium-doped fiber amplifier (EDFA) is commonly used in transoceanic cable transmission. However, EDFAs are generally too expensive for use in high-density metropolitan area networks (MANs). Erbium-doped waveguide amplifiers (EDWAs) have many of the advantages of EDFAs, are more economical to use in MAN applications, and yield better price/performance ratios than EDFAs in MANs. An EDWA comprises an erbium-doped waveguide embedded in a glass substrate. Its similarity to an EDFA derives from its use of an erbium-doped waveguide as a gain medium.
Referring to FIG. 3, U.S. Pat. No. 5,982,973 discloses an erbium-doped planar optical waveguide comprising a substrate, a bottom layer formed on the substrate, an active guiding layer arranged on the bottom layer, and a top cladding layer arranged over the active guiding layer. The sputtering deposition method is used for creating the active guiding layer and top cladding layer of the waveguide, and the material for the active guiding layer and top cladding layer is glass.
However, a planar waveguide amplifier is not suitable for optical communications because of its high polarization dependence. Furthermore, it produces optical amplification in all directions within a plane, rather than simply in the required linear direction.
Referring to FIG. 4, U.S. Statutory Invention Registration (SIR) H 1,848 discloses a Z-propagating waveguide laser and amplifier device in which a rare earth-doped lithium niobate (LiNbO3) crystal is used as a waveguide substrate. The waveguide is formed in the LiNbO3 crystal substrate, substantially parallel to the crystallographic Z-axis of the LiNbO3 crystal substrate. A metal diffusion method is used to create the graded refractive index of the crystalline waveguide.
Unfortunately, the metal diffusion method of U.S. SIR H 1,848 results in the waveguide laser and amplifier device having a gradual gradient distribution of rare earth ion density. There is no distinct boundary between the waveguide and the substrate. A stepped gradient distribution that matches the dimensions of corresponding input and output fibers cannot be attained.
In addition, the waveguide devices in U.S. Pat. No. 5,982,973 and SIR H 1,848 both take a lot of time to produce, and are relatively expensive. A suitable material is desired to lower the costs of optical amplifiers.